Collets are normally used in lathe machines for holding work stocks, usually bar of rods stocks, for machining. A collet comprises a tubular body having a plurality of work gripping segments at one end. On the outside radial surface of the gripping segments is formed a frustoconical surface. The collet fits into the spindle of a lathe. The spindle has a frustoconical surface which cooperates with the frustoconical surface on the work gripping segments. When the collet is moved relative to the spindle, the work gripping segments will either close and grip the work stock or open to release the work stock. Spring means for spreading the work gripping segments when pressure is released is usually provided in the body of the collet.
In a high speed production type lathe, the opening and closing action of the collet is performed with high frequency. This contributes to the breakage in the spring means in the collet body. When this occurs the whole collet has to be drawn out of the machine which could be a time consuming procedure. Breakage also occurs on the work gripping segments requiring the same lengthy replacement procedure as when the breakage occurs in the body of the collet.
To overcome the disadvantage described above, collets were later designed such that the work gripping fingers are separate from the collet body. With this improvement, replacement of the work gripping segments due to breakage could be carried out without taking the whole collet from the machine. But in one particular design, described in Peterson, U.S. Pat. No. 3,961,800, the spring means was retained in the collet body, thus still requiring the removal of the collet body from the machine when breakage occurs in the spring means. Another example is described in Parsons, U.S. Pat. No. 3,539,193, using removable work gripping pads and sealing members.
Subsequent improvement on the two piece collet resulted in incorporating the spring means in the work gripping segments themselves. An example of this design is disclosed in Peterson, U.S. Pat. No. 4,309,041. It discloses the use of springs and rubber seals in the work gripping segments in such a way that when pressure is released on the work gripping segments, the springs tend to urge the work gripping segments apart. Another example of this particular device is described in Rall, U.S. Pat. No. 4,214,766. This describes the use of volcanized rubber bonded to the work gripping segments in the same manner such that they will urge the segments apart when pressure on the collet is removed. The disadvantage of this particular design is that when the volcanized rubber wears out, the whole collet head has to be shipped to the manufacturer for replacement of the rubber material.